A Solid State NMR and EXAFS Study of Manganese Oxide Minerals

Abstract

Though only a minor component of soils occurring as thin coatings on rocks and sediments, manganese oxide minerals are excellent sorbents of various cations and have a significant impact on the geochemical cycling of environmentally relevant cations such as Pb, Cd, Ni, Cu and Zn. The layered manganese oxide, birnessite, has been found to have the highest sorption capacity of these Mn minerals, attributed to the negative layer charge that arises from structural "defects" such as layer Mn<super>3+</super> ions and Mn<super>4+</super> vacancies, as well as the high interlayer surface area available for the sorption of charge compensating cations. Great interest in understanding the nature of cation adsorption in these materials has resulted in numerous cation sorption studies, utilizing techniques to extract information regarding locations and ordering of sorbed cations and quantifying layer Mn vacancies. However, commonly utilized techniques such as EXAFS and X-ray diffraction have high errors associated with the refinement of cation coordination and quantification of layer vacancies, and require correlation of results from other techniques to support these refinements. Application of solid state NMR to these materials has not been extensively explored due to difficulties associated with the paramagnetic manganese ions. This dissertation aims at illustrating the utility of this technique in these paramagnetic materials. NMR is sensitive to the local environment, including the types and oxidation states of coordinated metals, coordination geometry and bonding distances surrounding the observed nucleus. <super>23</super>Na NMR is used to determine sodium locations with respect to the ordering of Mn<super>3+</super> and Mn<super>4+</super> in the triclinic form of birnessite and other layered sodium manganese oxides, indicating layer charge distribution drives cation ordering. <super>2</super>H NMR is used to understand the interactions of crystalline interlayer water with that of layer vacancies in hexagonal birnessite, and to quantify layer vacancies that are charge compensated by deuterons in these materials. Development of a special tailored structural model of Mn K-edge EXAFS data for these hexagonal birnessites support the trends observed in the <super>2</super>H NMR. The use of solid state NMR in these paramagnetic systems enriches our understanding of the complex interactions between the Mn layers and interlayer species in these important manganese oxide minerals.